The residual dynamics of a hypertidal coastal sea - a cognoscente´s perspective

The dynamics (currents, pressures/elevations, waves and density field) of Liverpool Bay, Irish Sea, have been measured and modelled continuously since August 2002 in an integrated programme. The Bay has strong tidal mixing, receives fresh water from several estuaries and stratifies intermittently. The measurements include 600 kHz ADCPs sampling the water column in 1 m bins at two sites in water 25 m deep (one with active sand-waves), fast sample ADVs and a phased array HF radar system, for surface currents and waves. Hydrodynamic models, on 1.8 km and 180 m grids, and a WAM wave model are run operationally and are currently being coupled. At high spring tides the tidal range can reach 10 m. The circulation is persistent, driven by the density field, whilst the wind driven (surge) currents are less important than in some shelf sea regions. There is significant energy at tidal frequencies (presumably non-coherent) in the tidal residual currents, derived from harmonic analyses, also present in the pressure records, which is being investigated by band-pass filtering to see whether the cause is predominantly shifts in the amplitude or the phase of the tides. The feasibility of assimilating the current information into the models is being tested.